Ethanol, an alcohol derived primarily from the fermentation of sugars, is a sustainable, low cost and environmentally-friendly energy alternative to fossil fuel. It is compatible with current and future transportation systems and results in near-zero net greenhouse gas emissions. The sugars required for the fermentation process may be obtained from multiple sources, and any substance capable of being treated to yield fermentable sugars may be considered a potential feedstock for ethanol production. These potential feedstocks include, but are not limited to, starches, plant matter (also called biomass) and food matter.
In some cases the feedstock may be a waste product from another process, such as wood pulp from paper-making or corn-stover from food crops. In other cases feedstocks may be grown specifically as a source of raw material for ethanol production.
Interest in ethanol as a fuel source is not new. In 1896 Henry Ford built his first automobile, the Quadricycle, to run on ethanol. A Model T designed in 1908 contained a carburetor adjustment that could allow it to run on ethanol. The design proved unsuccessful however, as high ethanol taxes led people to favor gas.
Demand for ethanol fluctuates with the rising and falling availability of oil, peaking during times such as World Wars I and II or the energy crisis of the 1970s. In the 1980s, U.S. Congress enacted a series of legislative measures aimed at making ethanol more attractive to potential producers. Today, as concern over the depletion of fossil fuels grows, interest in ethanol as a fuel source is again on the rise. As of January 2005, there were 87 U.S. ethanol plants with a total production capacity of 3,557 million gallons per year. Another 16 plants with a total production capacity of 681 million gallons per year are under construction.
Increasingly, farmers are investigating the value of adding an ethanol production facility to their operations. The benefits to the rural community in terms of job growth and economic opportunity are many. However, most ethanol production today is being done at large centralized processing facilities, because the methods currently used are economically viable only at capacities of 30 million gallons per year or more. There exists a need for more cost-effective ethanol recovery processes that will allow ethanol production to occur at the local level. Such production will bring benefits not only locally, but nationally and globally.
Interest in the production of other light organic compounds, for example acetone and other light alcohols, by fermentation is also increasing. In particular, the use of butanol as a biofuel or fuel additive is of interest.
In some cases, the raw materials that comprise the feedstock used to produce bioethanol may not contain any or sufficient fermentable sugars, so various conversion techniques may be used to obtain fermentable sugars, particularly from cellulosic or lignocellulosic materials. A recent survey of such approaches is given in Mosier et al., “Features of Promising Technologies for Pretreatment of Lignocellulosic Biomass,” Bioresource Technology 96, pp 673-686, 2004.
Early work on recovering ethanol from lignocellulose-containing material is described in U.S. Pat. No. 1,323,540, to Brown. The process includes a saccharification step, in which cellulose is broken down to sugars by acid hydrolysis. Numerous other patents describe hydrolysis of cellulosics by acid hydrolysis, enzymatic hydrolysis or other methods. Representative patents include U.S. Pat. Nos. 3,990,945; 4,174,976; 4,409,032; 5,628,830 and 6,333,181.
A recent innovation, known as simultaneous saccharification and fermentation (SSF), combines saccharification and fermentation into one step. U.S. Pat. No. 4,009,075, to Bio-Industries, describes a representative SSF process.
Recovery of ethanol from fermentation broth has been performed traditionally by filtering out solid material, and separating ethanol from water by distillation. U.S. Pat. No. 4,326,036, to Hayes, describes a process for manufacturing ethanol from sugar cane in which a membrane separation step is used to separate ethanol from water by pervaporation, and in which the ethanol-rich permeate vapor from the membrane separation step is passed for purification to distillation/rectification.
U.S. Pat. No. 6,755,975 to the U.S. Environmental Protection Agency and Membrane Technology and Research, Inc., describes a process for separating organic compounds from water using a combination of pervaporation and dephlegmation. The process can be applied to ethanol/water separation. U.S. Pat. No. 6,755,975 is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,899,743, to Membrane Technology and Research, Inc., described a process for separating organic/organic mixtures using a combination of membrane separation and dephlegmation.